The European Community is unique among international organizations in having a complex and highly developed system of internal law which has direct effect within the legal systems of its member states. Unlike other bodies of international law, it is generally recognized that European Community law overrides the national laws of its member states.

There are three sources of Community law:

primary legislation: the treaties

secondary legislation: regulations, directives and decisions made by the Community institutions in accordance with the treaties

The primary legislation, or treaties, are effectively the constitutional law of the European Union. They lay down the basic policies of the Union, establish its institutional structure, legislative procedures, and the powers of the Union. The treaties that make up the primary legislation include:

Secondary legislation also includes interinstitutional agreements, which are agreements made between European Union institutions clarifying their respective powers, especially in budgetary matters. The Parliament, Commission and Council are capable of entering into such agreements.

The classification of legislative acts varies among the First, Second and Third Pillars.

In the case of the first pillar: Secondary legislation is classified based on to whom it is directed, and how it is to be implemented. Regulations and directives bind everyone, while decisions only affect the parties to whom they are addressed (which can be individuals, corporations, or member states). Regulations have direct effect, i.e. they are binding in and of themselves as part of national law, while directives require implementation by national legislation to be effective. However, states that fail or refuse to implement directives as part of national law can be fined by the European Court of Justice.